This invention relates to purification of fullerenes, especially endohedral fullerenes, by chemical methods.
At the present time, endohedral fullerenes are available in much smaller quantities than the empty fullerenes. The major production method used to generate macroscopic quantities of empty fullerenes and endohedral fullerenes is the carbon-arc discharge method first revealed by Krätschmer and Huffman (Krätschmer, 1990). This method produces a carbon soot containing approximately 10–20% fullerene materials, the majority of which is C60 and C70. Of this 10–20%, only ca. 1% of the fullerenes are of the metallo-endohedral variety. Because the empty and endohedral fullerenes are formed together as an intimate mixture, it is necessary to first separate them from each other before investigations of their individual properties can proceed.
Chemical aspects of endohedral metallofullerenes and their purification have been the subjects of several published review articles (Bethune, 1993; Nagase, 1996; Liu, 2000, Shinohara (2000)). Many scientific papers and reports concern the purification and separation of endohedral metallofullerenes from empty fullerenes.
The great majority of these papers and reports involve chromatographic techniques, particularly high performance liquid chromatography (HPLC). Three of the key papers describing the first HPLC purifications of endohedral metallofullerenes are Shinohara et al. (Shinohara, 1993) for Sc2@C74, Sc2@C82, and Sc2@C84, Kikuchi et al. (Kikuchi, 1993) for La@C82, and Yamamoto et al. (Yamamoto, 1994) for La@C82. The most productive HPLC protocols now use dual-stage chromatographic procedures; a recent report detailing this is Okazaki et al. (Okazaki, 2000) highlighting the HPLC separation of Sm@C2n species. This dual-stage HPLC procedure uses two commercially available HPLC columns in series, the “Buckyprep” (Nakalai Tesque, Japan) and the “Buckyclutcher” (Regis Chemical, USA) columns, both of which are special columns developed for the separation of fullerene compounds. Meyerhoff (Xiao, 1994) reported that a derivatized triphenylporphyrin-silica HPLC stationary phase was efficient for the one-stage separation of La@C82 and Y@C82 from empty fullerenes.
Various solvent extraction procedures have been devised by which the content of endohedral fullerenes are enriched relative to the empty fullerenes. Typically these procedures use polar solvents to extract higher ratios of endohedral fullerenes relative to the empty fullerenes in as-produced fullerene soot. Examples of solvents used and endohedral fullerenes extracted include:                N-N-dimethylformamide for M@C82 (M=Ce, Ding, 1996; M=Gd, Sun, 1999)        Aniline for M@C60 (M=Y, Ba, La, Ce, Pr, Nd, Gd, Kubozono, 1996)        Pyridine for mixed M@C2n (M=La, Ce, Liu, 1998).Similar solvent extractions have been conducted prior to HPLC separations of endohedral metallofullerenes to increase the efficiency of the chromatographic operation. Tso et al. (Tso, 1996) reported a non-chromatographic procedure for enrichment of mixed Scm@C2n. A solid phase extraction with C18-bonded silica was performed on a chlorobenzene solution of empty fullerenes and Scm@C2n; 20 to 30% of the empty fullerenes were selectively removed by this technique.        
Sublimation techniques that partially enrich the endohedral fullerene content of mixed fullerene materials have been reported. Yeretzian et al. (Yeretzian, 1993) reported a gradient sublimation method for partial enrichment of La@C82. Diener et al. (Diener, 1997) conducted a variable temperature sublimation study of La@C2n and U@C2n. Cagle et al. (Cagle, 1999) reported a dual-temperature sublimation enrichment of HOm@C82. Ogawa et al. (Ogawa, 2000) recently reported a vacuum-sublimation enrichment of Er@C60.
The preparation of discrete ionic fullerene compounds in condensed media has been extensively reviewed (Reed, 2000(a)). This review covers the relevant literature from the discovery of the macroscopic production of fullerenes (ca. 1990) to the present. Synthesis and full experimental details for the isolation of salts of the fullerene cations C76+ (Bolskar, 1996; Bolskar, 1997), C70+ (Bolskar, 1997), C60+ (Bolskar, 1997; Reed, 2000(b)) and HC60+ (Reed 2000(b)) were reported as indicated. Tumanskii et al. (Tumanskii, 1998) reported the in-situ detection of proposed H(La@C82+) and H(Y@C82+) species.
U.S. Pat. No. 6,303,016 to Diener et al. discloses a method for extracting small bandgap fullerenes from a mixture based on reducing the small bandgap fullerenes until they can be solvated. Once dissolved, the small band gap fullerenes can be recovered by returning them to their charge neutral state. In a similar method, the electrochemical separation of the “small-bandgap” fullerene C74 from other empty “large-bandgap” fullerenes by a reductive electrochemical procedure was performed (Diener, 1998). In that procedure, the anion C74− was selectively produced as it reduces at a more positive potential than most of the empty fullerenes, due to its unusually high electron affinity. The C74− anion was then separated from the neutral fullerenes and finally plated out as a neutral species on a platinum electrode by bulk electrochemical single-electron oxidation. This procedure was also demonstrated in conjunction with endohedral Gd-metallofullerenes, producing a material enriched in Gd@C2n and C74 but deficient in large-bandgap empty fullerenes. The basis for this procedure is the notably higher first reduction potential of C74 (and Gd@C2n) as compared to the empty fullerenes. In principle, a similar reductive protocol could be conducted to separate endohedral metallofullerenes from empty larger-bandgap fullerenes or to separate fullerenes larger than C70 from the highly abundant C60 and C70.
Some time after the isolation of discrete fulleride (reduced fullerene) species, reversible electrochemical detection of empty fullerene cations (or fullereniums), i.e., fullerenes with one electron removed from the HOMO level of the neutral molecule, was achieved. This required more rigorous conditions and different solvents than those needed for the electroreduction experiments. Because the HOMO levels of common, empty fullerenes are relatively low-lying in energy, the fullerene cation species formed by removal of an electron from this HOMO are highly electrophilic. This property makes the fullereniums highly reactive toward nucleophiles. For these reasons, fullerene cations are very reactive toward even trace quantities of solvent impurities like water, so very high solvent purity and strictly water free conditions are preferred for their production. Preferred solvents are non-nucleophilic and weakly coordinating. Chlorinated alkane and arene solvents were found to be suitably non-reactive towards the transient cations produced in the electrochemical experiments. One-electron oxidation potentials for the empty fullerenes are summarized in the E1Ox column of Table 1. Note that the most abundant fullerenes C60 and C70 are more difficult to oxidize than the scarcer higher fullerenes. The high oxidation potentials for oxidation (greater than ca. +0.7 V vs. Fc/Fc+) confirm that these fullerene cations are highly reactive species.
Since the timescale of electrochemical generation and detection of fullerenium ions is on the order of seconds, these results did not guarantee that fullerene cations could be prepared and isolated as discrete salts. Fullerenium cation synthesis is a greater synthetic challenge than the isolation of discrete fulleride anions. Bolskar et al. (Bolskar, 1996) state “the problem of oxidizing fullerenes to fullerene cations is the problem of finding an oxidant strong enough for the task, but one which does not bring along with it a reactive nucleophile.” The oxidant must have enough thermodynamic driving force to abstract an electron from the neutral fullerene. Once accomplished, the now reduced oxidant must not subsequently react with the newly formed fullerene cation. The oxidant reagent also needs to deliver a counter-anion to balance the newly formed positive charge of the fullerene cation. This anion also must be inert and not react with the fullerene cation in any manner neither by nucleophilic attack and addition nor by electro-reducing the fullerene. The first synthesis of a fullerenium cation salt, that of [C76+][CB11H6Br6−] (Bolskar, 1996), is illustrative of these principles. C76 was chosen as the first test case for fullerene oxidation because its oxidation potential is lower than that of C60 (and C70) and it is commercially available as the pure D2 isomer. The synthesis of [C76+][CB11H6Br6−] proceeded in the following manner (Equation 1):C76+[(2,4-Br2C6H3)3N+][CB11H6Br6−]→[C76+][CB11H6Br6−]+(2,4-Br2C6H3)3N  (1)
The oxidizing agent in this case was the radical cation of tris(2,4-dibromophenyl)amine. With an E° of +1.16 V, it has sufficient oxidizing power to remove an electron from C76 (E1Ox=+0.81 V). The neutral amine reduction by-product is very weakly basic for steric and electronic reasons and does not react with the newly formed fullerene cation. The hexabrominated “carborane” [CB11H6Br6−] anion of the oxidizing reagent, which becomes the counterion to C76+, is perhaps one of the least nucleophilic and least reactive anions known (Reed, 1998) and does not react with the fullerenium ion. The hexahalogenated carborane anions ([CB11H6X6−], X=F, Cl, Br, I) have been shown to be exceptionally weakly nucleophilic and weakly coordinating such that a variety of extremely electrophilic cations once thought impossible to isolate can now be synthesized. An excellent example of this is the recent synthesis (Reed, 2000(b)) of the fullerenium radical cations C60+ and C70+ via a very strong oxidant paired with the hexachlorinated carborane counter-anion, [CB11H6Cl6−]. The new solid superacid [H+][CB11H6Cl6−] incorporating this exceptionally inert anion was also synthesized and used to protonate C60 (Reed, 2000(b)). The stable and fully characterized compound [HC60+][CB11H6Cl6−] is the first example of an isolated protonated fullerene species. These accomplishments of extreme oxidation and protonation, i.e., the synthesis of C60+ and HC60+ in condensed media, serve to demonstrate that fullerenes can in fact be protonated and oxidized and that the resulting cationic fullerene species are stable and can be handled and isolated under appropriate experimental conditions.
There remains a need in the art for improved methods for purifying fullerenes, especially endohedral fullerenes. The present invention provides a new, alternative purification strategy that exploits strong chemical property differences between the empty and endohedral.